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An Introduction to Flask A Lightweight Web Framework for Python

By David Li on 2025-01-31T19:30:20.000Z

An Introduction to Flask: A Lightweight Web Framework for Python

Flask is a popular and easy-to-use web framework for Python that allows developers to create web applications quickly and efficiently. It is a micro-framework that follows the “batteries-included” philosophy, providing a minimalistic core with the ability to extend its functionality using third-party libraries. In this article, we will explore the basics of Flask and demonstrate how to create a simple web application.

What is Flask?

Flask is an open-source web framework written in Python. It was created by Armin Ronacher as a lightweight alternative to more complex frameworks like Django. Flask is built on top of two key components: Werkzeug, a WSGI (Web Server Gateway Interface) utility library, and Jinja2, a powerful templating engine for Python. These components, combined with Flask’s simplicity and flexibility, make it an excellent choice for building web applications in Python.


To get started with Flask, you’ll first need to install it. You can use pip, the Python package installer, to install Flask:

pip install Flask

This command will install Flask along with its dependencies, such as Werkzeug and Jinja2.

Creating a Simple Flask Application

To create a basic Flask application, follow these steps:

  1. Create a new file named app.py in your desired directory.
  2. Open the file in your preferred code editor and add the following code:
from flask import Flask

app = Flask(__name__)

def hello_world():
    return 'Hello, World!'

if __name__ == '__main__':

In this code snippet, we start by importing the Flask class from the flask module. Next, we create an instance of the Flask class, passing it the name of the module or package, in this case, __name__.

The @app.route('/') decorator maps the root URL (’/’) to the hello_world function. When a client makes a request to the root URL, the hello_world function will be executed, returning the string ‘Hello, World!‘.

Finally, the if __name__ == '__main__': block ensures that the application only runs when executed directly, not when imported as a module. The app.run() function starts the built-in development server, which listens for incoming requests.

  1. Save the app.py file and open a terminal or command prompt in the same directory.
  2. Run the Flask application with the following command:
python app.py

You should see output indicating that the development server has started:

* Running on (Press CTRL+C to quit)
  1. Open a web browser and navigate to the URL You should see the text ‘Hello, World!’ displayed on the page.

Congratulations! You have just created your first Flask web application.

Extending Your Flask Application

Flask’s simplicity lends itself well to small projects and prototypes, but you can also build more complex applications using additional features and third-party extensions. Some popular extensions include:

  • Flask-SQLAlchemy: Adds support for SQLAlchemy, a powerful Object Relational Mapper (ORM) for working with databases.
  • Flask-WTF: Simplifies the process of working with forms and form validation.
  • Flask-Login: Provides user authentication and session management.
  • Flask-RESTful: Makes it easy to build RESTful APIs with Flask.

To learn more about these extensions and others, visit the Flask Extension Registry.


In this article, we’ve introduced Flask, a lightweight and easy-to-use web framework for Python. We demonstrated how to create a simple web application and touched on some of the possibilities for extending Flask with third-party libraries. By combining the simplicity of Flask with the power of Python, you can quickly build and deploy web applications to suit your needs.

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